1. Field of the Invention
This invention relates to a communication apparatus and, more particularly, is applicable to a digital cellular telephone apparatus.
2. Description of the Related Art
In a digital cellular telephone apparatus, an apparatus for communicating by a time division multiple access (TDMA) system in which a communication channel is multiplexed by dividing the same radio channels has been used. In such a digital cellular telephone apparatus, communication with a base station is performed by using a time-divided communication channel, and so transmission and reception must be performed alternately in time.
For instance, in the standard of digital cellular telephone apparatus in Japan, as shown in FIG. 1, one slot has been set to about 20 mS, and in that 6.7 mS is set as transmission (T), 6.7 mS is set as reception (R), 1 mS is set as a level measurement term (LM) of antenna city reception, and 5.7 mS is set as an idling term (I) for measurement of a receiving level from neighboring base stations.
In such a digital cellular telephone apparatus, a battery is used as a source of power. A consumption current of this battery varies in time because transmitting operation and receiving operation are performed alternately as described above. For instance, a consumption current about 1.2 A flows in transmission, and a consumption current about 0.3 A flows in reception.
Generally, voltage drop arises corresponding to a consumption current because a battery has internal resistance, as a result, a battery voltage output from the battery varies. For instance, in the case where a consumption current varies as described above, a battery voltage in transmission become smaller than that in reception.
Internal resistance of a battery tends to become higher by repeating charge and discharge. Thus in using of such a battery that has sequentially repeated charge and discharge, it is feared that a battery voltage becomes lower than a reset voltage at the moment of transmission started and the apparatus is reset because the internal resistance is extremely high; conversation is suddenly cut off.
To avoid this, heretofore, a method has been provided: a battery voltage in reception is measured to uniformly predict a battery voltage in transmission based on the measured value, and as a result, if thus predicted value is lower than a reset voltage, an alert is sounded to inform battery dead to a user and conversation is ended.
However, a power amplifier operating in transmission requires a large consumption current, and besides, one which fluctuates. In the method described above, there is a possibility that in some apparatuses even if a predictive value is higher than a reset voltage, a battery voltage become lower than a reset voltage owing to the fluctuation of a power amplifier on a practical transmitting operation; suddenly the apparatus is reset and conversation is cut off without sounding an alert. Further, in the above method, if the internal resistance increases by repeated charges and discharges of a battery, the above problem will arise.